etherion

from The Century Dictionary.

• noun A supposed new element announced by C. F. Brush in 1898, and described by him as a gas of density only one-thousandth that of hydrogen and having a much greater conducting power for heat, as present in the earth's atmosphere, and as perhaps extending throughout interstellar space. Sir W. Crookes has shown that etherion is probably nothing more than highly rarefied vapor of water.